The present invention relates to a diagnosing system for an engine and, more particularly, to a diagnosing system for an engine suitable for diagnosis in a direct-injection engine in which fuel is injected directly into combustion chambers or a lean-burn engine.
Techniques for using a lean mixture having an air-fuel ratio greater than the theoretical air-fuel ratio, i.e., the stoichiometric air-fuel ratio, have become prevalent with the progressively increasing severity of environmental protection regulations and a growing tendency for environmental protection to reduce the fuel consumption of engines. Gasoline engines are classified into those of the port injection system which injects a fuel into the suction port to supply an air-fuel mixture of an air-fuel ratio in the range of about 20 to about 25 for lean-burn combustion and those of the direct fuel injection system (hereinafter referred to as "cylinder injection system") which injects a fuel directly into the combustion chamber to supply a very lean air-fuel mixture having an air-fuel ratio in the range of about 40 to about 50. The fuel consumption of the lean-burn engine is small because pumping loss and thermal diffusion in the lean-burn engine are low.
The port injection system, for instance, promotes the mixing of fuel and air by positively forming swirls of intake air by an intake air flow intensifying means, such as a swirl forming valve, to stabilize lean combustion. The cylinder injection system localizes the distribution of the fuel in the cylinder so that fuel concentration of the air-fuel mixture around the spark plug is increased by positively producing air flow by properly determining fuel injection timing, using intake air flow intensifying means, such as a swirl control valve or a tumble control valve, and properly determining the shape of a cavity over the piston to enable very lean combustion.
The port injection system supplies a lean air-fuel mixture to the engine for lean combustion in an operating mode requiring relatively low output, and supplies a stoichiometric or rich air-fuel mixture to the engine in an operating mode requiring high output. The cylinder injection system injects the fuel into the cylinder of the engine for stratified combustion in an operating mode requiring relatively low output, and injects the fuel into the cylinder of the engine so that a homogeneous air-fuel mixture is produced in the cylinder for lean combustion using an air-fuel mixture having an air-fuel ratio in the range of about 20 to about 25, stoichiometric combustion or rich combustion in an operating mode requiring higher output. The port injection system supplies a homogeneous lean air-fuel mixture or a homogeneous stoichiometric air-fuel mixture according to the operating condition of the engine. The cylinder injection system supplies a stratified lean air-fuel mixture, a homogeneous lean air-fuel mixture or a stoichiometric air-fuel mixture according to the operating condition of the engine.
Lean burning is realized by an air-fuel mixture supply means including the intake air flow intensifying means and the fuel supply means. If those means do not function properly, unstable combustion occurs. If unstable combustion occurs, part of the fuel does not burn, the raw fuel is discharged and the injurious gas concentration, such as Co and NOx concentration, of the exhaust gas is liable to increase. If the injurious gas concentration of the exhaust gas discharged from the engine is extraordinarily high, the exhaust gas purifying means, such as a catalytic converter, included in the exhaust system is unable to purify the exhaust gas satisfactorily. Consequently, an increased amount of injurious gases is emitted into the atmosphere, vibrations are generated due to torque variation, the catalyst is burnt due to the burning of the unburned gas in the catalytic converter, and fuel consumption rate increases. Regulations require the diagnosis of a malfunction which increases injurious gases abnormally by an on-vehicle control unit. Such regulations requiring self-diagnostic operations are enforced currently in the U.S.A. and the enforcement of such regulations are under consideration in Europe and Japan.
A malfunction detecting technique, such as a technique for diagnosing combustion state including misfiring, is disclosed in Japanese patent No. 2,559,509. This technique estimates a combustion state on the basis of the variation of engine speed.
There have been disclosed many other techniques including a technique which estimates a combustion state from an ion current that flows between electrodes placed in a combustion chamber, a technique which estimates a combustion state from combustion pressure in the combustion chamber measured by a combustion pressure sensor placed near the combustion chamber, and a technique which estimates a combustion state from the output torque of the engine.
Although those known techniques are able to detect the deterioration of the combustion state due to, for example, misfiring, the same are unable to identify the malfunction of the intake air flow intensifying means and the fuel supply means. Therefore, other detecting means must be added to the engine or the engine must be examined by engineers at a maintenance shop spending much time.
When the fuel is supplied by the cylinder injection system for stratified combustion, the fuel is distributed in the cylinder in an unexpected distribution if the fuel is injected by a fuel injection valve in a spray condition greatly different from a desired spray condition or the intake air flow intensifying means malfunctions, and a large amount of unburned gas is discharged even if combustion is stable. If such a malfunction occurs in a specific cylinder among a plurality of cylinders, combustion pressures in other cylinders and torque produced by the same decrease slightly. Therefore it is possible to detect the malfunction by the conventional technique. However, it is difficult to discriminate between an abnormal condition and a normal condition because the different cylinders are by nature different from each other in operating condition. It is difficult to detect a subtle malfunction. Because different engines have different characteristics and different parts, and the condition of the engine changes with time.
The present invention has been made in view of those problems in the conventional techniques and it is therefore an object of the present invention to provide a diagnosing system for an engine capable of diagnosing malfunctions in an intake air flow intensifying means and a fuel supply means without being affected by difference in characteristics between different engines, difference in parts and the change of the condition of the engine with time, and of specifying the cause of the malfunction.